A reliable operation of transformers requires sufficient electrical insulation as well as the dissipation of the heat released during the conversion of electrical voltages. It is known that certain fluids have insulating and heat-dissipating properties. Conventionally, mineral oils or silicones are used. However, they have very poor biodegradability and thus represent a hazard for humans and the environment in the case of leaks, defects in liquid tightness or another discharge from the transformer. Mineral oils in addition have a very low flash point below 150° C., i.e., a high fire hazard potential.
Therefore, readily biodegradable plant oils have been proposed for use as insulation fluid in transformers. It is obvious to use plant oils as insulation fluid, since they are readily and completely biodegradable and generally not hazardous for water (according to the German “Administrative Regulation on Substances Hazardous to Waters”—VwVwS) and they have flash and fire points above 300° C. (according to the method by Pensky-Martens), all this at advantageous raw material costs. In addition, these plant oils have a higher water absorption capacity than mineral oil, which reduces the degradation of the cellulose of the transformer board and increases the useful life of the transformer.
Plant oils have already been used as insulation oils approximately since the end of the 19th century. However, their use was soon discontinued, since they resinify relatively rapidly by oxidation when air enters the transformers in which they are used. As a result of the use of hermetically sealed transformers, which largely exclude the entry of air, the requirement profile has changed in recent years.
The oxidation sensitivity continues to be important, but not to the extent it was in the transformers of the past, and it is manageable in hermetically sealed transformers. On the other hand, awareness about the environment has increased considerably worldwide. Accordingly, plant oils such as castor bean oil, sunflower oil, rapeseed oil, soybean oil and other oils have been proposed a number of times as transformer fluid, see also WO 97/22977 A1 and U.S. Pat. No. 6,340,658 B1.
In addition to oxidation stability, other required properties of a transformer fluid have become increasingly important, including high flash and fire points, low viscosity (for improved heat convection), and in particular also a low pour point, low acid number, good dielectric stability and low sludge formation in the stability test according to DIN EN 61099 “Specifications for unused synthetic organic esters for electrical purposes” (see Table 1). In addition, good corrosion properties and seal compatibility are absolutely required. Unfortunately, natural plant oils do not satisfy all these necessary or desired properties simultaneously, and they have weaknesses in terms of one or more of the properties, in particular with regard to viscosity and cold properties as well as oxidation stability. The oxidation stability is generally increased to a minimum level by adding antioxidants. However, the cold properties in particular can only be improved marginally by means of additives. Lowering the viscosity by simply mixing plant oils with portions of other clearly thinner base oils is not possible because of required high flash and fire points.
GB 1602092 discloses the use of trimethylolpropane esters of linear saturated fatty acids with 7 to 10 C atoms and their use as dielectric insulation fluid for transformers. From the examples, trimethylolpropane esters having a viscosity of 25 or 30 mm2/s in each case at 30° C. and a fire point of 277° C. or 293° C. are known. WO 2005/118756 A1 has a similar disclosure content. However, it discloses more broadly linear or branched carboxylic acids with 6 to 12 C atoms. However, branched carboxylic acids are not natural fatty acids.